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You shouldn't pack-unpack-pack-unpack too many times, especially if you have set the quality low to minimize the size of the JPEG file.
But then again: Some people judge the quality of compression (whether photos, video or sound) solely based on the file size - disregading both the software creating the file, and the method used, taking for granted that bigger file = better quality. For "natural" shapes, JPEG is actually quite good.
Note one very important thing about JPEG (that also holds for MP3 and AAC audio and MPEG/H.26x video): Compression is not standardized. Decompression is! Two JPEG files may contain very different data streams, both decompressing to very similar expanded images. Two compressors may use very different strategies for creating a data stream that will decompress to the desired result. Simple software just find "something that works"; more advanced software may try out different alternatives, do the decompessing and see how much it differs from the uncompressed input image, and select the encoding that minimises the differences. Or set parameters to reduce losses below a given treshold.
The basic idea of JPEG is that with a point light source illuminating a flat surface, the brightness will vary over the surface by a cosine function. With a distant, "flat" light source like the sun, a spherical surface will receive ligth varying with the cosine of the angle between the light source and the surface normal. A matte (non-blank) surface reflects light in given direction as a cosine function of the angle to the direction of the light. An opaque material, such as a white lamp dome, spreads light in a similar way.
So, cosine distriubtions are very common. A photo of a smooth ball illuminated by a point source (or by flat light) could in theory be reduces to a handful of number describing the intensity and color of the light source, the size and reflectivity of the ball. These numbers are what a JPEG compressor strives to find.
Photos of smooth balls are not that common, so the image is split into quite small squares that "locally" is like a section of a close-to-spherical surface. The first approximation is to assume that it is part of a sphere, and determine, from the distribution of tones, the radius and a possible light source. For e.g. cheek sections of a portrait, even the first try may come very close to the input image. In other sections, like around the eyes, lips etc., the compressor must select the most dominant spherical surface, and then add another surface the same way, so that when the two are added, they come closer to the original. To get even closer, a third elmemnt can be added, a forth, and so on. If you end up with a discrepancy from the original less that the value of the least significant bit (i.e. if the pixel value are integers from 0 to e.g. 2**8 and the packed and unpacked values differ by less than 0.5), you are actually loosing nothing, yet the series required to represent this may be more compact than raw encoding of every pixel.
If a picture contains elements far from natural cosine-friendly curvatures, but e.g. have lots of sharp transitions, then you must continue that series with many elements to reduce the discrepancies. But a good compressor will do that only in those parts where it is required: "Easy" picture parts receive only a small part of the bit budget, to leave more for the difficult parts. It could lead to e.g. the cheek being slightly smoother than in the original image, to make the text parts sharper.
Note that JPEG in fact tries to describe a limited-pixel-resolution image using continous functions over those square picture fragments. If it succeeds, with sufficientl small artifacts, it has in fact recreated an analog, resolution independent model of each fragment, that could in principle be resampled in any resolution before display (assuming that the resampling is done directly on the cosine functions, not on the unpacked raster image).
So, while it is advisable to use an uncompressed format while editing and adjusting the photo, you should not "be afraid of" JPEG as the final format when all work is done - provided that you use a high quality compressor. Lots of people who insist on lossless formats, whether sound or image, fail miserably in blindfold tests where they do not know anything about the file size, method used etc. and are limited to watching the images at a normal viewing distance, without using a magnifier, and without doing a diff between two images. Even if that is granted them, they may be unable to tell which is the original and which is "ruined by compression artifacts".
I used to think that but then I found an image handling program with an adjustable quality factor and I no longer do. Many programs, like mspaint, have a fixed quality factor on their exporter and it is set so low that it results in nasty artifacts. With an adjustable QF, I find that a quality factor of 90 to 95% results in an image where I can't see any differences from the original and it still gives an good compression ratio. When I stay with the PNG format too often I get an image with virtually no compression so it is pointless.
I definitely see value in keeping images in their original format if you are going to continue to work with it. Successive compressions can compound the loss and it adds up. I do a lot of work with the DDS or DXT format and the loss can be painfully obvious there.
"They have a consciousness, they have a life, they have a soul! Damn you! Let the rabbits wear glasses! Save our brothers! Can I get an amen?"
A "bitrate" of 96 - do you by that mean 3*32 bits per pixel? I've never seen a scanner that has that color depth! Usually it is either 3*8 = 24 or 3*12 = 36 bits.
If the slides are underexposed, and you use the same exposure for all of them, you will only use a fraction of the density range in any case. All the scanners I have been working with allows the exposure to be set in the scanner. If all those dark shadows are more or less stretched out to a 3*8 bit/pixel range before they leave the scanner, the color resolution is likely to be more that good enough, at least for amateur use.
If you are going to do extensive post-processing, you might want to use a 12 bit format for the working copies - but even though jpeg defines a 3*12 bit format, the support for it is far from universal. "Quite low" would probably be a good description. So when you are through processing, you should probably save it in 3*8 format.
To capture 3*12 bits from the scanner, you will usually control the scanner from your photo editor, which must be capable of handing 3*12. The image is usually transferred from the scanner to the editor in uncompressed format (I think that my current scanner isn't even capable of returning scans in JPEG format!), and it is up to the photo editor to select a working storage format, which may be similarly uncompressed.
Generally speaking, repeated cycles of JPEG unpacking - photo editing - JPEG packing is not a good idea. If you are going to do that, at least make sure that you set the JPEG quality to maximum while working with a photo, even though the JPEG files will grow in size. Also, scan at the maximum optical resolution that your scanner provides - but note that some scanners claim a much higher resolution than what is real, by interpolating between the actual scan values. I have seen scanners with an optical 1200 dpi resolution deliver an interpolated 9600 dpi resolution. This you might as well do on your PC - maye in a much better way. That depends on your software; some scaling functions do a quite decent job of e.g. identifying sharp edges and preserving that in the interpolation process.
Slides are a good idea if they are shrap, exposed correctly and displayed on a high quality screen by a high quality projector in a "home movie theater". But since you get no opportunity to make up for incorrect exposure during printing (because there is no printing!), you depend on an automatic exposure camera - which was non-existent in the 1950s) or a photographer who knew how to use a light meter.
Maybe there existed cameras in the 1950s with built in light meters, but they were few and far between (if they existed at all). For the great majority of cameras well into the 1960s (or even later), you set the shutter speed and aperture according to the instructions on the box that the film came in: In bright sun, use f/8 and 1/250 sec. In open shadow use f/5.6 and 1/125 sec, and so on. That is, if your camera was fancy enough to have adjustable shutter speeds and aperture. My first camera, an Instamatic 50 (from the very first series of Instamatic cameras made by Kodak - I guess it would have a high price on the collecor's market today!), had a slider for "Sunny" or "Cloudy", that was all.
A possible reason for your slides being "quite dark" might be that The High Quality Slide Film was - more or less throughout history - the ISO 25 Kodachrome, requiring five times as much light as the ISO 125 Plus-X monchrome negative film. I believe that the Kodacolor color negative film went from ISO 80 to ISO 100 during its lifetime. A photographer putting a Kodachrome in the camera without noticing that the aperture would have to be increased by more than two f-stops (i.e. half the value, e.g. from f/8 to f/4), or the shutter speed adjusted similarly might end up with consistenly underexposed, dark slides.
I've never seen a scanner that has that color depth!
It probably has only 16-18 significant bits, I did work with a some imaging equipment with that bit depth. The light sensitive part of the high end X-Ray detectors can push up to 23 bits, and it's basically a scanner (X-Ray sensors are normal light scanners coated with a scintillator that emits light when struck by X-Rays).
GCS d--(d+) s-/++ a C++++ U+++ P- L+@ E-- W++ N+ o+ K- w+++ O? M-- V? PS+ PE- Y+ PGP t+ 5? X R+++ tv-- b+(+++) DI+++ D++ G e++ h--- r+++ y+++* Weapons extension: ma- k++ F+2 X
My Dad (who took the images) was a camera buff (in the 1950's) and yes he bought my Mum one of the first Kodak Instant Cameras(?) I can recall my Dad's frustration when 'they' stopped making the film for it! The images were instant but faded over time...
I had, and I am so happy that it has been shelved now. Editing photos on the screen is a thousand times more convenient than the old style darkroom work.
Kodak did make a try at instant film and cameras, but they were not very successful in beating Polaroid. Polaroid was The instant camera manufacturer, both before and after Kodak's attempts.
There weren't any great selection of chemical processes to choose from for instant photography. For "plain" photography, there were more. If you pick up an old photo album, you may see some of the prints almost completely faded out, while others on the same page have kept up the colors. If you then find the envelopes with all the negatives, you will probably see that the faded ones were printed by one photolab, the good ones by another.
Monochrome doesn't fade out to the same degree: The black in the photo isn's a dye, but silver in metallic form. Silver is quite stable. Far better that any dye.
Kodak's Disc format was not any great success. Now that I check Wikipedia I am surprised to learn that there were other manufacturers as well - I never saw anything but quite cheap Kodak cameras for Disc film.
I do not think that the format deserved much more success - a frame size of 10 by 8 millimeters is just too small. And the large size of the disc put constraints on camera production. The circular arrangement of the frames means that all photolabs need to invest in equipment whith completely different mechanics for handling the film - it is not just a narrower or wider strip of "linear" film, you cannot splice strips together to pull through processor as a continous strip etc. You cannot put it into a standard enlarger in an amateur darkroom (I don't think they made any monchrome Disc film anyway; darkroom amateurs usually didn't handle color).
"Dark" Note that after a certain level, there's nothing in shadows but mud. You'll actually have lower resolution in the shadow. ( Which is to say, more resolution probably won't help there.) But, one of slides advantages is a much longer range - light to dark. The detail may be there.
Most of the simple photo editors - I like IrfanView - will let you adjust brightness and contrast.
First kick both. ( Tweek - check - repeat. Check at close and far. ( or different magnifications ) )
You may want to adjust the contrast or brightness curve ( more brightness and contrast in shadow... ), which some programs allow. The Gimp and Photoshop ( and ?? ) will let you dodge and burn, and use unsharp masks ( "I need more contrast in that area" ). Much of what they do and how follows what can be done in a darkroom.
If you're trying to save precious images, go with RAW at maximum bit depth.
As other said you can enhance them with either GIMP or Photoshop, there is also a cool free image processing software called ImageJ which is mostly aimed to those who perform mathematical imaging but can read literally any format and do any kind of operations on images.
GCS d--(d+) s-/++ a C++++ U+++ P- L+@ E-- W++ N+ o+ K- w+++ O? M-- V? PS+ PE- Y+ PGP t+ 5? X R+++ tv-- b+(+++) DI+++ D++ G e++ h--- r+++ y+++* Weapons extension: ma- k++ F+2 X
Let's face it, there is a lot of bad advice here. Basically you should consider 2 formats when scanning: JPEG which uses a lossy compression and TIFF where you may use one of the lossless compressions. Consider TIFF as the archival format at 16 bits/colour/pixel. JPEG, besides the compression, has the disadvantage to store only 8 bits per colour channel. That's enough for a final picture but not one where you intend to do still some adjustments. with TIFF you are filling up your disk space.
So now scanning: Best is to use the scanner software's possibilities to do a basic adjustment of your scan, like histogram adjustments. As with your digital camera, if you get the settings right immediately with the scan you will be ok. If you want the data to be archived, what is my understanding of this, then you need to use the best physical quality your scanner can do. Use dust removal carefully and it's best when there is hardware support for this.
If dust removal is done by software, you're best using your post-processor (GIMP, Photoshop, Affinity Photo, Corel Paintshop (?),...) for this. Your Post-processor should be able to work non-destructively. Use colour management through the chain to stay consitent with your colours. The best profile to use is ProPhoto RGB. The final JPEG, however, should be saved as sRGB. If your scanner and post-processor is not able to handle colour profiles then you are using the wrong tool.
The whole process, until the final JPEG for viewing/printing/fast access takes a lot of time. You shouldn't be in a hurry for the scan job or the clean-up job.
Your quality is going to depend upon your scanner to a large part. Jpeg is fine if your initial scan is good. Try using Vuescan software. It will allow you to adjust the brightness at the time of scan. This will reduce your detail loss better than any file format option.
Next if things still seem to dark or washed out then I would suggest the software package LightZone not to be confused with LightRoom by Adobe. LightZone is free and has the ability to lighten and darken certain zones of light. I have had wonderful results with this software.
Imagine a world without computers, without monitors where you had photographic prints and pretty lousy resolution television pictures. For a relatively small amount of money you could purchase a fold up screen and a projector and blow up your photos to 3' x 4' or larger.
They were ubiquitous. Often you just had to show up for a business presentation with your tray of slide and you were good to go. Everyone had projectors and screens on site.
They brought terror into homes all across America as the dreaded invocation was heard. "We'll fix some snacks and you can see slides of our trip across the Rockies and from San Diego to Vancouver with two of the cutest little poodles in the world!"
I know you have the hardware, but my experience back a few years ago was that under $2000 equipment just didn't cut it. Over that price I didn't try.
And the scanning services were not up to snuff except for one.
I found ScanCafe and ended up doing 10 to 12 thousand slides, some photos, and a tiny number of 8mm movies from the 30's over the course of a couple years. The whole thing was a rare, rare excursion in to very-happy-customer land for me.
They regularly have sales where the price for one media or another is way-lower than normal. Get on their email list and wait a few months to save 1/3 to a half. My slides were in the 20 cent range, some more, some less.
I sent them 50 or 100 or so slides as a test before boxing thousands of slides. The test runs were done on high importance slides that had been scanned other ways. ScanCafe's scans were clearly superior.
Answer to the parenthetical question: Slides were the only way to go back in the day. Printed pics were flat, comparatively speaking. Also, a lot of pics just work better at a big size in the distance. But that's like, do you want to see your pics on a 75 inch monitor or on a phone? Glowing or dim? Life size or tiny?
I don't know how long "a few years ago" is on your scale. Nor do I know your ambitions. But I have a feeling that you are in the same league as those who think that it has no value to digitize worn, scratchy vinyl records unless you do it at minimum 96 kHz sampling rate (preferably 192 kHz) at minimum 24 bit sample with. Anything less will result in garbage sound, no matter how worn and torn the old vinyls are...
Five years ago, you could buy quite cheap scanners that in reality preserved everything that was possible to preserve from badly expose, badly preserved amateur photos. Even for correctly exposed and well preserved amateur photos! Tenyears ago, the scanners were as good, but moderately priced, not "cheap". I bought my first film scanner more than twenty years ago - I don't remember its price tag, but it didn't have significant effect on our family budget. With high ISO films, I could see the silver grains. (The reason why I gave that scanner away is that I said farwell to SCSI interfaces.)
Those who care about old-time photography should look up some old issues of "Modern Photography" and "Popular Photography" from the 1960s and 70s, with special attention to the lab test of both lenses and film. By modern standards, the resolution was shockingly low. The dynamic tone range was "quite limited" too, to phrase it politely. Even though some films boasted an exposure latitude of 3-4 f-stops (i.e. a factor of 10-50), the essential requiement at the end of these ranges were that you were able to recognize what the camera was pointed at. Noone expected a soft tone scale at the outer end of the exposure range.
To be blunt, we are talking about "throwing pearls before swine" if we require more than 9600 bpi spatial resolution and at least 24 bits per channel. Setting up such requirements will lead to a lot of people just throwing away those old historical photos. Scanning them at 1200 dpi at 12 bits, or even 8 bits, per channel, is far better than not scanning them at all! Fact is that noone will conmplain about the quality! (except those who look at the metadata for the image file, claiming that is it not up to their technical standards).
I agree with all you say. My mantra is, "First, get it digitized and let the bits take care of themselves." I have a lot of phone or old point-and-shoot camera digitizations of paper based stuff. Why? Because just clicking a camera is quick and easy and the picture is phenomenal, if begging for post-processing ... someday.
My slide scan-o-thon was from 2014 to ~2016.
ScanCafe slide scans were notably better than the various other methods I tried. Keep in mind I can't physically perceive the kinds of nuances the committed "X-phile" (or even average-aged person) can sense. Just getting noise, dirt, and color-balance in half-way decent shape is worth it, though. Too, scanning thousands of slides can take serious time. Rankling though it was to spend roughly the same nominal money on digitization as I had on the original Kodak slides, there was no way I could do it burning my own time. YMMV.
It might also be that the scanner software isn't allowing enough adjustment.
I'd recommend VueScan (www.hamrick.com). It allows numerous adjustments for scanning, or a simple mode if preferred. It also supports older scanners that the vendor (Canon in my case) no longer support on Windows 10 with their official software. It is not free, but there is a free trial.
I've used it to scan a number of slides I inherited from my father, though none of them were poorly exposed so I didn't use any of the available tweaks.
It's been a while, but if I recall it also could handle scanning and cropping multiple slides in one pass.